A cellular radiotelephony network, such as GSM for example, contains several base radio stations to which mobile terminals are connected. The landline telephone network connects the base stations, and the latter guarantee radio coverage of corresponding cells, which are overlapping to avoid communication outages when mobile terminals change cells.
When users turn on their mobile terminals, the terminals are required to connect to the base station of the corresponding cell. For this to occur, a message indicating presence is sent through the signaling channel, called a control channel, also indicating field levels received from the stations within radio range, including information concerning the identity of the stations. The network processes the message, and connects the terminal to the station receiving the strongest signal. The terminal is thus registered in the mobility chart of a nodal point controlling mobility, called BSC (Base Station Controller). The charts of different BSCs thus represent the position of various turned on terminals, whether in use for communication, or idle. Calls from other terminals to a terminal can thus be routed within the network, with reference to the mobility charts.
The term cell is understood to refer to a broadcasting space positioned around the station in which broadcast waves of the station are, on the one hand, located above a specified field level, and on the other hand, above the field level received from other cells of the network.
When the terminal switches from one cell to another, the level of the wanted signal, received from the initial station, becomes significantly weaker than the one received from the station of the new cell. When the discrepancy between both levels exceeds the hysteresis threshold, the original cell erases the terminal from its charts, and the new cell records the signal. This transfer, in English, is called Hand-Over. Other than the identity of the terminal, the two cells are required to exchange specific data, and in particular data concerning the called party, if the terminal was in use for communication.
Other than the landline network, connecting base stations to traffic concentrators, in turn connected to the message switching network that handles all traffic from the radio terminals, there is also a parallel signaling network, designed to administer equipments, base and other stations of the radio network, depending on service data, or signals exchanged between cells to administer terminal mobility.
The signaling network is often integrated with the other network, and in fact consists of circuits, or reserved time paths. The signaling network thus mobilizes a certain percentage of resources, and it is thus desirable to keep its size, in terms of circuits, at the lowest possible level compatible with the level of availability needed to transmit signals within required time constraints.